That's a common question... and unfortunately the answer is that "watashi wa tabemasu" and "tabemasu" are absolutely different from each other and you can't just go around substituting one for the other. You also can't really translate the difference between the two of them, but in context, a good translation might be:
Watashi wa tabamasu. -> As for me, I'm eating.
Tabemasu. -> I'm eating.
It bears repeating: this depends on context. But you can see how "watashi wa" sticks out like a sore thumb, and you can instantly recognize novice Japanese second-language speakers because they say "watashi wa" all the time. And you'll later learn that "watashi" is sometimes an inappropriate way to refer to yourself...
Watashi wa tabamasu. -> As for me, I'm eating.
Tabemasu. -> I'm eating.
It bears repeating: this depends on context. But you can see how "watashi wa" sticks out like a sore thumb, and you can instantly recognize novice Japanese second-language speakers because they say "watashi wa" all the time. And you'll later learn that "watashi" is sometimes an inappropriate way to refer to yourself...